swift234

Don't complain about missing witnesses if they're available in extensions.

(Specifically, unconstrained extensions.)

This removes the problem where trying to conform to a protocol spits out
a dozen errors about missing requirements in a base protocol, but eleven
of them have a sensible default implementation with no constraints. This
is "P6" in the test cases; the others all passed before.

There's still plenty more work in this space. In particular, if an
associated type is missing we just give up rather than encouraging people
to provide the members it would be inferred from.

Swift SVN r31224